There are a few things that could cause this error. I'd start by taking a Fiddler trace of signing out, so that you can see the assertions being sent between your client, the IdP, and Tableau Server. If you need help capturing/interpreting that trace, that is something Tableau Support can assist you with if you open a case.
totally agree. i have read the article you posted from previous reply. It did give me some new thought. Although i can see the XML logouRequest being sent from my idp server back to SP-tableau server, i can't be certain if the xml msg actually reached to tableau server or not. We got Digicerts for both tableau server and sso server though. So i am not sure if SSL can really cause the issue. I have added tableau cer in my idp's truststore. Do i also need to add idp cer in tableau...? ....